1. Field of the Invention
The present invention relates to a step-down switching regulator, and particularly to a control technique for a synchronous rectification switching regulator.
2. Description of the Related Art
In recent years, microprocessors for providing digital signal processing are mounted in various electronic devices such as cellular phones, PDAs (Personal Digital Assistants), notebook-sized personal computers, etc. The power supply voltage necessary for driving such a microprocessor is being reduced as the fine semiconductor manufacturing process is being improved. For example, a microprocessor is known which operates at a low voltage of 1.5 V or less.
A battery such as a lithium ion battery or the like is mounted on such electronic devices as a power supply. The lithium ion battery outputs voltage of around 3 V to 4V. Such an arrangement, in which the output voltage is directly supplied to the microprocessor, leads to wasteful power consumption, and accordingly, in general, after the battery voltage is stepped down using a step-down switching regulator, a series regulator, or the like, the constant voltage thus stepped down is supplied to the microprocessor.
Two types of step-down switching regulators are known. One is a switching regulator using a rectifier diode (which will be referred to as a “rectifier diode switching regulator” hereafter). The other is a switching regulator using a rectifier transistor instead of the rectifier diode (which will be referred to as a “synchronous rectification switching regulator” hereafter). The former type has the advantage of exhibiting high efficiency when a low load current is applied to a load. However, such an arrangement requires a diode, in addition to an output inductor and an output capacitor, in the form of external components to a control circuit, leading to a large circuit area. On the other hand, the latter type provides poor efficiency when a low current is supplied to the load, as compared with the former type. However, with such an arrangement, a transistor is employed instead of a diode, which allows the control circuit to be integrated in the form of an LSI. This offers a small circuit area incorporating peripheral components. There is a demand for reducing the size of electronic devices such as cellular phones. In many cases, a switching regulator using a rectifier transistor (which will be referred to as a “synchronous rectification switching regulator” hereafter) is employed in such an arrangement in order to satisfy such a demand for a reduced size.
Directing our attention to the microprocessor employed in the aforementioned electronic devices, when the microprocessor operates for performing computation processing, a certain amount of current flows through the microprocessor. On the other hand, when the microprocessor is in the standby state, only a small amount of current flows through the microprocessor. FIG. 8A is a diagram which shows the current waveform with respect to time when the synchronous rectification switching regulator is connected to a heavy load. FIG. 8B is a diagram which shows the current waveform with respect to time when the synchronous rectification switching regulator is connected to a light load. In these drawings, IL represents the current that flows through the output inductor (which will also be referred to as the “inductor current IL” hereafter). Iout represents the load current. Here, the load current Iout is obtained by averaging the inductor current IL over time. As shown in FIG. 8A, when the synchronous rectification switching regulator is connected to a heavy load, the load current Iout is large. Accordingly, the inductor current IL is always positive. Here, the inductor current IL flowing toward the load is positive by definition. On the other hand, let us consider a case in which the synchronous rectification switching regulator is connected to a light load as shown in FIG. 8B. In this case, reduction of the load current Iout leads to a negative inductor current IL as indicated by the hatched portion in FIG. 8B. That is to say, in this stage, the inductor current IL flows through the output inductor in the reverse direction. As a result, with such an arrangement employing the synchronous rectification method, when the synchronous rectification switching regulator is connected to a light load, current flows from the output inductor to the ground through the synchronous rectifier transistor. This current is supplied from the output capacitor, but is not supplied to the load. This leads to wasteful power consumption.
For example, Patent documents 1 through 3 disclose switching regulators each of which has a function of switching rectification methods between the synchronous rectification method and the diode rectification method based upon the load current. In the techniques described in Patent documents 2 and 3, the inductor current IL is monitored. In a case in which the inductor current changes from a positive value to a negative value, the synchronous rectifier transistor is turned off, thereby improving the efficiency.
[Patent Document 1]
Japanese Patent Application Laid-open No. 2004-32875
[Patent Document 2]
Japanese Patent Application Laid-open No. 2002-252971
[Patent Document 3]
Japanese Patent Application Laid-open No. 2003-319643
Conceivable examples of arrangements having a function of detecting the direction of the inductor current include: an arrangement in which a resistor is connected to the output inductor in series, and the voltage between both terminals of the resistor is monitored; and an arrangement in which the voltage at the node between the switching transistor and the synchronous rectifier transistor (which will be referred to as the “switching voltage Vsw” hereafter) is monitored. FIG. 9A is a time chart which shows the switching voltage Vsw when a light load is connected to the switching regulator. As shown in FIG. 9A, when the light load is connected to the switching regulator, the switching voltage Vsw is the high-level state during a period of time Tp1 in which the switching transistor is in the ON state. Next, during a period of time Tp2 in which the synchronous rectifier transistor is in the ON-state, the switching voltage Vsw temporarily becomes negative, following which the switching voltage Vsw gradually increases as the inductor current IL is reduced. Subsequently, the switching voltage Vsw becomes 0 V at the point in time when the direction of the inductor current IL reverses (which will also be referred to as the “zero-crossing point” hereafter). Using this mechanism, the light load state can be detected by making a comparison between the switching voltage Vsw and the threshold voltage Vth (=0 V). During a period of time Tp3, both the switching transistor and the synchronous rectifier transistor are in the OFF state.
In general, a comparator is employed in order to make a comparison between the switching voltage Vsw and the threshold voltage Vth. The comparator has a limited response speed. Accordingly, the output signal of the comparator changes after the elapse of a delay time ΔT from the point in time when the magnitude relation between the two comparison-target voltages changes. FIG. 9B shows the output signal Vcmp of the comparator that makes a comparison between the switching voltage Vsw shown in FIG. 9A and the threshold voltage (=0 V). Here, when the switching voltage Vsw is greater than the threshold voltage Vth, the output voltage Vcmp is in the high-level state. On the other hand, when the switching voltage Vsw is smaller than the threshold voltage Vth, the output voltage Vcmp is in the low-level state.
With an arrangement in which the threshold voltage Vth is set to the fixed value 0 V, during the period of time Tp1 in which the switching transistor is in the ON state, the output voltage Vcmp of the comparator is in the high-level state. When the switching transistor is turned off, the switching voltage Vsw becomes smaller than the threshold voltage Vth. Subsequently, the output signal Vcmp of the comparator becomes low-level. Specifically, the transition in the output signal Vcmp requires a delay time Δt. With such an arrangement, if the period of time τ from the point in time when the switching transistor is turned off and the synchronous rectifier transistor is turned on, up to the point in time when the direction of the inductor current IL reverses is smaller than the delay time Δt, the comparator cannot detect the zero-crossing point.